In extracorporeal blood treatment, blood is taken out of a patient, treated and then reintroduced into the patient by means of an extracorporeal blood flow circuit. Generally, the blood is circulated through the circuit by one or more pumping devices. The circuit is connected to a blood vessel access of the patient, typically via one or more access devices, such as needles or catheters, which are inserted into the blood vessel access. Such extracorporeal blood treatments include hemodialysis, hemodiafiltration, hemofiltration, plasmapheresis, etc.
US2005/0010118 proposes a technique for monitoring a patient's pulse rate, blood pressure and also the condition of the blood vessel access, by identifying a frequency component of the pressure wave caused by the patent's heartbeat among other pressure waves in the extracorporeal blood flow circuit, by operating a frequency analysis, such as a Fourier transformation, on a pressure signal obtained from a pressure sensor in the extracorporeal blood flow circuit. As noted in US2005/0010118, it might be difficult to extract the relevant frequency component from a mixture of frequency components caused by mechanical devices in the extracorporeal blood flow circuit and by the heartbeat. In particular, the frequency component of the heartbeat may overlap with a frequency component of the mechanical devices. To overcome this limitation, US2005/0010118 proposes e.g. changing the frequency of the blood pump within a certain range of a basic operating frequency during the treatment procedure. The pressure signal from the pressure sensor in the extracorporeal blood flow circuit is analysed by FFT (Fast Fourier Transform), which is not suited for detection of frequency components whose frequencies are constantly changing. The FFT analysis is alleged to reduce the frequency components caused by the blood pump. However, periodic events caused by other mechanical devices in the extracorporeal blood flow circuit, such as valves, may still interfere with the monitoring. Further, it may be undesirable to operate the blood pump with a constantly changing pumping frequency during the treatment procedure. For example, if the extracorporeal blood flow circuit is part of a dialysis machine, the dialysis dose will decline with changed pumping frequency even at unchanged average flow through the extracorporeal blood flow circuit.
Thus, there is a need for an alternative technique for identifying the patent's heartbeat among other pressure waves in a fluid, and in particular a technique with an improved ability to handle the situation when the frequency of the patient's heartbeat is relatively weak and/or at least partially coincides with a frequency component of these other pressure waves and/or is changing over time.
Corresponding needs may arise in other fields of technology. Thus, generally speaking, there is a need for an improved technique for processing a time-dependent measurement signal obtained from a pressure sensor in a fluid containing system associated with a first pulse generator and a second pulse generator, in order to monitor a functional parameter of the fluid containing system by isolating a signal component originating from the second pulse generator among signal components originating from the first and second pulse generator.